Solder reflow heater bar assembly

ABSTRACT

Heater bar assembly for a reflow solder machine comprises first and second fingers (50, 52) carrying U-shaped heater bar (54) which is pivotable on main member 22. A spring urges the heater bar assembly to the centered position. First and second thermocouples (82, 84) are secured to the heater bar (54) and are connected through a transfer switch (104) by which their electrical connections can be alternated. One controls the power supply and the other cuts off the power supply if temperature exceeds the selected temperature.

BACKGROUND

This invention is directed to an easily interchangeable assembly whichcarries a solder reflow heater bar so that the heater bar can pivot withrespect to the work to apply an even force over several leads beingsoldered, and is also directed to an associated control system whereintwo thermocouples on the heater bar respectively sense the heater bartemperature to control input power and to sense over-temperature to stopthe heating cycle when over-temperatures are sensed.

In reflow soldering, a printed circuit board carries pads thereon whichare tinned with solder. A component, often with many parallel leads, isput in place with its leads over the pads. A heater bar is brought downinto contact and heat is applied. Lack of compliance of the heater barcauses uneven force upon the leads to result in unreliable soldering.Free pivoting of the heater bar does not control the initial angle ofthe heater bar and this results in lateral motion of the heater bar asit moves down into soldering position. Such lateral motion may swing theleads off of their pads, may cause the leads to contact adjacent pads,or may cause limited lead-path contact with consequent unreliability.

A thermocouple is mounted on the heater bar to sense the heater bartemperature and transmit this sensed data to the power supply. It can bereadily determined that the thermocouple has failed by becoming an opencircuit, and such detection can control the power supply circuitry todisable the power output when the thermocouple circuit is open. However,present power supply circuitry cannot detect partial or total shortcircuits in the thermocouple circuit. If this mode of failure occurs inwhich the thermocouple resistance has deteriorated to a value betweennominal resistance and short circuit, with circuit resistance less thannominal, there is a corresponding lower signal amplitude which resultsin excessive heating, which often causes damage to the printed circuitboard and heater bar.

Heater bars are subject to degradation and destruction and occasionallymust be changed. Thus, it is also necessary to provide a heater barassembly which permits quick and easy changing to minimize productiondown time. Accordingly, there is need for a solder reflow heater barassembly and control system which is easily changed, which is properlycompliant with the level of the leads to be soldered, and is arranged sothat thermocouple failure can be sensed before excessive heating of theboard takes place.

SUMMARY

In order to aid in the understanding of this invention, it can be statedin essentially summary form that it is directed to a solder reflowheater bar assembly and control system wherein the heater barsubassembly is readily demountable and is pivoted and biased toward acentral position. The heater bar carries two thermocouples which areconnected to alternately control the power supply and sense excessivetemperatures.

It is, thus, a purpose and advantage of this invention to provide asolder reflow heater bar assembly which greatly improves the reliabilityand quality of reflow soldering with extended longevity of the heaterbar assembly.

It is a further purpose and advantage of this invention to provide twothermocouples on the heater bar, with the thermocouples connected toprevent catastrophic thermal damage to the printed circuit boards beingsoldered, in the event of failure of one of the thermocouples.

It is a further purpose and advantage of this invention to greatlyreduce the time required to replace the failed portion of the assemblyin order to resume production.

It is another purpose and advantage of this invention to reduce theamount of lateral force and reduce the distance of lateral motion of theheater bar with respect to the component leads as the heater bar isbrought down to greatly reduce component skewing in the solderingprocess so as to yield fewer defects in part orientation and solderdisturbance.

Other purposes and advantages of this invention will become apparentfrom a study of the following portion of the specification, the claimsand the attached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an isometric view of the solder reflow heater bar of thisinvention associated with parts to be soldered and schematically showingthe reflow solder machine in dashed lines.

FIG. 2 is an enlarged front elevational view of the solder reflow heaterbar assembly, with parts broken away.

FIG. 3 is a right side elevational view thereof.

FIG. 4 is a schematic diagram of the circuitry of the control system.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Printed circuit board 10 is illustrated in FIG. 1. Mounted thereon issemiconductor device 12 which has a plurality of leads 14 extending fromat least one side thereof. The purpose of reflow soldering machine 16 isto solder the leads onto pretinned pads on the printed circuit board.Reflow soldering machine 16 includes a ram, which is moved downwardly tobring the reflow solder heater bar assembly down to the leads 14, powersupply 18 and power supply control system 20, see FIG. 4. Machine 16 hasa ram which moves toward the printed circuit board when soldering isdesired. To this ram is secured main member 22 by means of upper andlower machine screws which pass through slotted openings 24 and 26. Thepurpose of the slotted openings is to permit the alignment of the mainmember in a direction perpendicular to the printed circuit board 10 andparallel to the direction of motion of the main ram. Keyway 28 engageson a cooperating key on the ram so that even though there is lateraladjustability, the main member 22 exactly follows the motion of the ram.

Spring block 30 is fastened on the rear (as seen in FIG. 1) of mainmember 22. Spring block 30 is notched to engage around the sides ofmember 22 and is secured in place by means of screw 32 and a pair ofpins 34 and 36. Spring block 30 has hole 38 therethrough, see FIG. 2,and in the spring hole 38 lies tension spring 40. There is an upright,half cylindrical yoke in each end of spring block 30. Yoke 42 is shownat the left end of the block in FIG. 2, while yoke 44 is shown at theright of the block. These yokes extend past spring hole 38. Pins 46 and48 respectively lie in these yokes. A loop on each end of tension spring40 engages around one of the pins to resiliently urge the pins into theyoke recesses.

Fingers 50 and 52 double as supports and conductors for the wide,U-shaped heater bar 54 which is secured to both of them. The heater baris thin at its tip 54 so that most of the resistance heating occurs attip 54. The fingers lie next to each other over a substantial part oftheir length, but as is seen in FIG. 3, finger 52 thins to a blade 56which supports the front of the heater bar, while finger 50 thins to ablade 58 which supports the back of the heater bar. The fingers are ofelectrically conductive material such as a copper alloy, while theheater bar is a U-shaped sheet metal structure made of a metal which hasa higher electrical resistance and greater resistance to solder wetting,tinning or other degradation in this service. Inconel is suitable. TheU-shaped heater bar is brazed to the blades for security.

Pivot block 60, see FIGS. 2 and 3, is positioned between fingers 50 and52. Pivot block 60 has a key on each side thereof, one of which fitsinto a keyway in each of the fingers. Downward motion of pivot block 60is limited by the two blades 56 and 58. Upward movement is limited byleft and right pins through fingers 50 and 52. The right one isindicated at 59 in FIGS. 1 and 3. Pivot block 60 is made of insulatormaterial and has therein pivot recess 62 into which extends pivot pin 64which is mounted on the bottom of main member 22. In this way, the lugs,the fingers, the pivot block and the heater bar comprise the heatersubassembly that can rotate around the nose of pivot pin 64. Sufficientclearance is provided in pivot recess 62 and between the fingers 50 and52 to permit relative motion. Insulator disc 66 is mounted on the lowerend of main member 22 to prevent either of the fingers from rocking intocontact with the main member to provide electrical isolation and torestrain the heater subassembly from moving downward.

Saddle block 68 is made of insulator material and spans the width of thefingers. Lug 70 is on the end of lead 72, while lug 74 is on the end oflead 76. These lugs have slots in the end thereof with the upper legs ofthe lugs above the slot extending toward each other above disc 66 butnot touching each other. When in place, as shown in FIG. 2, they holdthe heater subassembly up. Screws 78 and 80 pass through openings in thesaddle block, through the slotted openings in the lugs and into threadedopenings in the respective fingers. In this way, the saddle block issecured in place and the leads are securely connected to the fingers.

In addition to the electrical leads, there are two thermocouple sensors82 and 84 brazed to the inside of the U-shaped heater bar. The leadsfrom the sensors pass through ceramic tube 86 which is mounted inV-grooves in the facing sides of blades 56 and 58. The four leads fromthe two thermocouples terminate in connector pins 88, 90, 92 and 94which extend outward from saddle block 68. These connector pins serve asquick disconnects for the thermocouple leads to the circuitry of controlsystem 20. Leads 96 and 98 carry springs 100 and 102 which resilientlywrap around the connector pins. Connection and disconnection can bereadily made by turning the spring with respect to the pin in the springunwinding direction.

This connection makes it easy to replace the detachable heatersubassembly. The thermocouple leads can be quickly removed. Thereupon,the screws 78 and 80 are loosened to permit the removal of the powersupply leads. The heater subassembly can now be lowered off of pivot pin64 and removed. Reattachment of a new heater bar assembly is similarlyconvenient.

Power supply 18 is part of the reflow solder machine 16. Itconventionally has therein a sensor which indicates when thetemperature-sensing thermocouple is an open circuit. The control system20 includes the power supply 18 and its built-in open thermocouplesensor. As is seen in FIG. 4, the thermocuples 82 and 84 are connectedthrough transfer switch 104. With the transfer switch in the positionindicated, leads 96 and 98 of thermocouple 82 are connected by leads 106and 108 to the open circuit sensor as well as to the temperature controlcircuitry in the power supply. Thus, thermocouple 82 is connected tocontrol the power supply to limit the temperature reached by the heaterbar tip 54. Relay 110 has normally closed contacts in the leads 106 and108.

Thermocouple 84 is connected to lines 112 and 114, and they areconnected to the inputs of amplifier 116. The output of amplifier 116 isconnected to SCR 118 which controls relay 110. Amplifier 116 is set sothat a slightly higher temperature at thermocouple 84 causes actuationwhich opens relay 110 to signal an open circuit to the power supply toshut down the power. Amplifier 116 is an Analog Devices thermocoupleamplifier. In this way, when the desired temperature at the heatingelement is exceeded, the power supply is shut off, even whenthermocouple 82 does not sense and signal the excessive temperature.

In order to prevent a situation where thermocouple 84 has an opencircuit and thus is unable to protect against low resistance inthermocouple 82, transfer switch 104 interchanges the functions ofthermocouples 82 and 84 with each cycle of the power supply. The powersupply is equipped with circuitry which energizes the transfer switch104 on alternate cycles. In this way, if either thermocouple is open ordecreases circuit resistance, it is detected during that cycle or thenext cycle.

In this way, a solder reflow heater bar subassembly is easilyinterchangeable and is properly compliant to the level of the leads onthe board. In addition, the control associated therewith permits promptdetection of thermocouple failure and promptly signals such failure.

This invention has been described in its presently contemplated bestmode, and it is clear that it is susceptible to numerous modifications,modes and embodiments within the ability of those skilled in the art andwithout the exercise of the inventive faculty. Accordingly, the scope ofthis invention is defined by the scope of the following claims.

What is claimed is:
 1. A heater bar assembly comprising:a main member, apivot on said main member; first and second electrically conductivefingers, a heater bar interconnecting said fingers, means on at leastone of said fingers for pivotally mounting said fingers on said mainmember; and resilient means interconnecting said fingers and said mainmember for resiliently urging said fingers with respect to said mainmember to a centered position.
 2. The heater bar assembly of claim 1wherein an insulator block is mounted on both of said fingers and saidpivot means is on said insulator block.
 3. The heater bar assembly ofclaim 2 wherein said pivot means between said main member and saidfingers comprises a pivot pin in a pivot recess.
 4. The heater barassembly of claim 3 wherein said recess is in said insulator block andsaid pivot pin is the convex end of a pin forming a portion of said mainmember.
 5. The heater bar assembly of claim 4 wherein said resilientmeans comprises a tension spring.
 6. The heater bar assembly of claim 5wherein there is a saddle block connected to both said fingers andinsulated with respect to said fingers and there are first and secondpins engaged by said tension spring, said saddle block being positionedbetween said pins.
 7. The heater bar assembly of claim 6 wherein thereis a spring block mounted on said main member, said spring block havingfirst and second ends and having first and second yokes respectivelyformed in said first and second ends, said pins lying within said yokesso that displacement of one of said pins from its yoke causes stretchingof said tension spring.
 8. The heater bar assembly of claim 7 whereinsaid saddle block is of insulated material and is secured to said firstand second fingers by first and second bolts, respectively, said boltsdetachably attaching first and second power leads to said first andsecond fingers.
 9. The heater bar assembly of claim 1 wherein saidresilient means comprises a tension spring.
 10. The heater bar assemblyof claim 9 wherein there is a saddle block connected to both saidfingers and insulated with respect to said fingers and there are firstand second pins engaged by said tension spring, said saddle block beingpositioned between said pins.
 11. The heater bar assembly of claim 10wherein there is a spring block mounted on said main member, said springblock having first and second ends and having first and second yokesrespectively formed in said first and second ends, said pins lyingwithin said yokes so that displacement of one of said pins from its yokecauses stretching of said tension spring.
 12. The heater bar assembly ofclaim 11 wherein said saddle block is of insulated material and issecured to said first and second fingers by first and second bolts,respectively, said bolts detachably attaching first and second powerleads to said first and second fingers.
 13. The heater bar assembly ofclaim 12 wherein there are first and second thermocouples secured tosaid heater bar, terminals for said thermocouples on said saddle block,said thermocouples being electrically connected to said thermocoupleterminals.
 14. A heater bar assembly comprising:a main member movabletoward and away from the work piece upon which a soldering step is to beperformed; first and second fingers and a heater bar electricallyinterconnecting said first and second fingers, said first and secondfingers being pivotally mounted with respect to said main member; meansresiliently interconnecting said fingers and said main member toresiliently urge said fingers and said heater bar carried thereon to acentered position with respect to said main member; first and secondthermocouples secured to said heater bar, said first thermocouple beingconnectable to a power supply to control the amount of electric powerdelivered to said heater bar to control the temperature of said heaterbar to a desired temperature level, said second thermocouple beingconnected to the power supply to stop the power supply when said secondthermocouple detects temperature above the desired temperature.
 15. Theheater bar assembly of claim 14 further including a transfer switchelectrically connected to said first and second thermocouples forelectrically exchanging said first and second thermocouples.
 16. Theheater bar assembly of claim 15 further including means for controllingsaid transfer switch so that it electrically interchanges said first andsecond thermocouples substantially on each actuation of said powersupply.
 17. A heater bar assembly comprising:first and second fingersfor electrical connection to a heater power supply, a heater bar mountedon said first and second fingers to be heated by current supplied tosaid fingers; first and second thermocouples mounted on said heater bar,said first thermocouple being connectable to the power supply to controlthe power supply to supply power until said heater bar reaches aselected temperature, said second thermocouple being connected to thepower supply to turn off the power supply when said second thermocouplesenses a temperature higher than the selected temperature; a transferswitch connected to both said first and second thermocouples to beactuated periodically by actuation of said power supply for transferringthe connections of said first and second thermocouples so that saidsecond thermocouple controls the power supply to deliver current until aselected temperature is reached and said first thermocouple turns offthe power supply when the selected temperature is exceeded.